Mixing processes with different stirring systems in agitated vessels are simulated by a multi-region model. Because of discontinuities at the boundaries of different regions appearing for the already-existing simple three-region model, unrealistic values Occur during the calculation of the concentration distribution at these locations. Therefore, the region model will be modified at first. The procedure is presented here for the example of a radial flow stirrer, considering the fluid velocity, the turbulence field, the particle diffusion, and the relative fluid-to-particle velocity. The modified model is discussed and validated. Thus, the behavior of single particles and particle swarms can be analyzed fit the vessel considering different aspects.